1. Field of the Invention
This invention relates to the preparation of homopolymers and copolymers of vinyl halide such as vinyl chloride having reduced grain size and melt viscosity, improved impact strength and easy processability. The polymers are useful in the production of films, coatings, and molded articles. Scale build-up is eliminated during bulk polymerization of the polymers of the invention.
The polymers of the invention have impact strength improved by as much as a factor of 20 over the unmodified prior art polymers. Additionally, the polymers of the invention can be more easily processed to produce molded articles, since the polymers of the invention have excellent mold release properties allowing molds to be made using the polymer of the invention without the usual additional lubricant components or the use of mold release agents which are used on the mold itself in order to provide a surface to which the polymer will not stick readily.
Fine grain homopolymers and copolymers can also be obtained by the process of the invention having narrow grain size distribution and 10 to 30 micron average grain size range, thus permitting the polymers of the invention to be used in applications where previously bulk polymerized polymers were unsuited because of their excessive grain size and wide grain size distribution. In prior bulk polymerization methods, polymers are produced having average grain sizes in the range of about 80 to about 150 microns. More costly suspension polymerization processes are presently utilized to produce fine grain size polymers.
Polyvinyl chloride has been produced by bulk or mass polymerizaton methods in the absence of solvents and diluents. The polymer produced has an irregular shape which may vary from minute globular particles to irregular, non-globular masses. The efficiency of the bulk or mass polymerization processes of the prior art is reduced by the unwanted production of "scale" which is defined as a resinous build-up or deposit on the surface of the reactor and on the agitator. The scale is a very hard fused resin and is generally produced in an amount between 1 and 5% of the total resin yield. Since the scale so produced cannot be used commercially, and additionally, it is necessary to expend time and effort to remove it from the equipment, such scale production detracts from the efficiency of the process. The term "lump" can be defined as resinous material formed during the bulk polymerization characterized as being a soft agglomerate. Lump formation in the usual bulk polymerization process amounts to between 3 to 4% of the total product. Although the lump resin can be used after grinding to produce small particle size resinous material, it would be better if this could be eliminated. In British Pat. No. 1,047,489 and U.S. Pat. No. 3,522,227, both of which are hereby incorporated by reference, there are described a method of polymerization in mass in two steps which produces an improvement in regularity of granular size and shape. The process is more controllable and the product more uniform than can be obtained by other methods of bulk polymerization. In this process, the monomer is polymerized in a first stage to about 7 to about 15 percent conversion, preferably, about 8 to about 10 percent conversion in a reactor in which provision is made for high speed, high shear agitation. At the end of the first stage polymerization, the mass is transferred to a second reactor and further polymerized using slow speed agitation to an appropriate end point (70 to 85 percent conversion in many cases). It is an object of this invention to produce polymers having a small particle size which remain substantially unsolvated when placed in contact with primary type plasticizers for vinyl chloride resins.
Vinyl halide polymers of the prior art have been prepared in a wide variety of molecular weights; those of high molecular weight generally having better physical properties such as hardness and strength than those of lower molecular weight. The higher the molecular weight, however, the more difficult it is to process the polymer into a final product form.
In the subsequent processing of vinyl halide polymers, such as milling and extruding, higher molecular weight polymers require the use of higher shear force and/or higher temperatures to fluidize the polymer in comparison to lower molecular weight materials. The increased shear force and/or increased temperature required increases the internal temperature of the polymer and tends to degrade the vinyl halide polymers. In addition, the increased shear force necessary to process the polymers adds to the cost of processing the polymer. It is an object of this invention to produce easy processing polymers having greater fluidity at elevated temperatures and reduced resistance to shear force without sacrificing other physical properties. Another object of the invention is to produce a polymer having excellent mold release properties. Conventional lubricants and mold releasing agents such as polyethylene wax, silicone resin, fluorinated resins, etc., which are added to resins of the prior art prior to molding, can be eliminated when the polymers of the invention are used.
Mechanical blends of aliphatic hydrocarbon olefin polymers and polyvinyl halide have been prepared in an effort to produce easy processing polyvinyl halide polymers. These mixtures are incompatible and unsatisfactory. Graft copolymers of polyolefins such as polyethylene, and vinyl chloride have been prepared in the presence of a free radical initiating compound using aqueous methods of polymerization, e.g. suspension polymerization, in an effort to increase the flexibility at low temperatures of polyvinyl halide polymers and copolymers as well as improve their impact resistance. These graft copolymers overcome the incompatibility of physical blends of polyolefins and polyvinyl chloride but these methods of producing graft copolymers are expensive and time consuming, and aqueous polymerization of vinyl halides is well-recognized in the art as being chemically distinctive from bulk vinyl halide polymerization in both process and product. It is a further object of the present invention to produce graft copolymers by the bulk polymerization of a polyolefin dissolved in a vinyl halide monomer. The process of the invention provides graft copolymers having unexpectedly large improvements in impact resistance for the amount of polyolefin polymer or terpolymer used and having a distinctive difference in the kind of polymer breakage which occurs on impact.
It is an additional object of the present invention to produce a vinyl halide polymer by a bulk polymerization process in which the tendence of the polymer to form scale deposits on the equipment is greatly reduced or eliminated by polymerization in the presence of an olefin polymer soluble or partially soluble in vinyl halide.